Fibre-cement boards, such as fibre-cement panels for roofings and siding boards are high-rate construction materials possessed of numerous advantages over other construction materials, such as, for example, wood panelling, polyvinyl chloride sheets or metal siding boards, made from aluminum or steel sheet, for example. One major advantage is the durability of fibre-cement sheets to weathering effects, and the mechanical stability, too, is oftentimes better than for other construction materials.
Fibre-cement boards are manufactured from cement, water, optionally with fillers, such as sand or silica, and of a fibre fraction comprising natural fibres, such as wood fibres or cellulosic fibres, for example, and also of synthetic fibres. The mixture is introduced into a mold, optionally dewatered on a sieve, and then cured. The raw mass may be cured as it is or else compacted by pressing in order to achieve a greater ultimate strength postcure. The fibre-cement boards may be manufactured as flat sheets or else as what are called corrugated or embossed sheets. The fibre-cement boards may be given a surface texture, by embossing, which mimics the appearance of, for example, a wood surface. The boards may be cut into a variety of shapes and sizes: large panels, small squares, rectangles or rhombuses, or else as shingle replicas or replicas of other building materials. The fibre-cement boards typically manufactured have a thickness in the range of about 3 to 20 mm.
Fibre-cement boards require curing. This is usually done in a humidified heat chamber with or without pressure. At temperatures above 30° C. to around 70° C., the curing of the cement takes place in an accelerated regime, within a few hours, to give a material having sufficient strength to resist being destroyed by crushing. Curing may also take place in an autoclave, under pressure, in which case the temperature may also be significantly higher, since water is unable to escape and it is not possible for the cement to “fire”. The water needed for the hydration of the cement does not evaporate. Autoclave curing is performed, for example, at a temperature in the range of 100° C. to around 180° C.
One disadvantage of fibre-cement boards, is that the effect of weathering, in particular the action of water, leaches out the cationic constituents such as Ca2+ over the course of time, lessening the strength of the components. A further deleterious property of mineral surfaces is the occurrence of efflorescence phenomena. These phenomena are presumably attributable to the polyvalent cations such as Ca2+, which are present, in the mineral binders, in an alkaline environment. Through reaction with the carbon dioxide from the air, it is possible in this way for unattractive white lime flecks, difficult to dissolve in water, to form on the surface of the fibre-cement boards. The efflorescence may appear not only during the hardening of freshly prepared fibre-cement boards but also, under exposure to weathering, on fibre-cement boards that have already hardened.
Following their manufacture and curing, often fibre-cement boards are coated with an aqueous dispersion or paint, which is based on an aqueous polymer dispersion, in order to provide the necessary protection against efflorescence. The coating can be applied, e.g. by dipping, spraying, rolling or brushing or by a curtain coater. In some instances, the back and the edges of the fibre-cement boards are coated as well. After coating and drying have been carried out, the fibre-cement boards are usually stacked on one another while still in a warm state. In such stacks, there may be instances of sticking of the coated sheets to one another, since the coating binders may be too soft and hence lacking sufficient blocking resistance.
Apart from this, coated fibre cement boards often show poor weathering resistance in terms of gloss retention, as UV radiation from sunlight results in loss of gloss due to degradation of the binder and/or yellowing. Moreover, humidity may cause an undesirable color change of the coating, in particular in terms of lightness and whitening. Humidity may also result in an undesirable water-uptake of the coating which reduces mechanical strength of the coating.
Typical binders for coating compositions for the surfaces of mineral moldings such as concrete slabs or fibre cement boards include styrene/acrylic ester copolymers, homopolymers and copolymers of vinyl acetate, straight acrylates, and the like (cf. e.g., DE 21 64 256). The coatings obtainable with these polymer dispersions, however, are unable to provide satisfactory prevention of the passage of the cationic constituents (efflorescence). Moreover, coatings of this kind readily become soiled.
EP 458144 discloses aqueous polymer dispersions based on acrylate and vinylaromatic monomers, which contain keto group containing monomers and photoinitiators, namely benzophenone type compounds. The polymer dispersions are suggested as binders for coatings such as latex paints for walls, floors and ceilings, in particular in architectural coatings or facade paints, respectively.
Protection to surfaces of mineral moldings from the above-described efflorescence has also been improved by coating materials based on styrene/acrylate dispersions or on straight-acrylate dispersions of EP-A 469 295 and of WO 96/33143, respectively. For this purpose, EP-A 469 295 recommends the use of a specific aromatic sulfonate based anionic emulsifier, while WO 96/33143 advises the use of polymers which comprise, in copolymerized form, specific monomers containing sulfonate groups. The water resistance of films made of polymers containing such ionic species is however diminished due to their hydrophilicity. In particular, the coatings show a considerable water-uptake.
EP 915 071 discloses coating compositions based on copolymers of ethylenically unsaturated monomers containing 0.2% to 5% by weight of itaconic acid. The coating compositions are suggested for coating mineral moldings such as concrete slabs or fibre-cement boards. EP 915 071 focuses on the coating of concrete slabs.
WO 99/46219 teaches the use of coating compositions for mineral moldings, which contain polymer dispersions based on alkylmethacrylates different from methyl methacrylate. The dispersions contain large amounts of emulsifiers based on aromatic sulfonic acids, such as bis-sulfonated diphenyl ethers, and focusses on the coating on concrete slabs.
WO 00/20355 discloses the use coating compositions for mineral moldings, which contain slightly crosslinked polymer dispersions based on alkyl(meth)acrylates. The dispersions contain large amounts of emulsifiers based on aromatic sulfonic acids, such as bis-sulfonated diphenyl ethers, and focusses on the coating on concrete slabs.
EP 1069093 discloses a method for coating mineral moldings, such as concrete slabs, comprises the subsequent application of at least two different coating compositions based on aqueous polymer dispersions. The aqueous polymer dispersions are similar to those described in WO 96/33143 or WO 00/20355.
DE 103 43 726 discloses coated concrete moldings featuring effective effloresence protection by an aqueous polymer dispersion and of an anionic emulsifier having at least one phosphate and/or phosphonate group.
EP 1 948 574 discloses a coated fibre-cement article having a crush resistant top coating based on a multistage latex polymer. The multistage latex polymer features a glass transition temperature (Tg) gradient.
WO 2012/084737 discloses multistage latex polymers containing a sulfonated acrylic ester or (meth)acrylic amide monomer of the and their use thereof in coating compositions for fibre-cement boards.
None of the polymer dispersions of prior art provide for coatings on fibre cement boards, which show a good water resistance, a good weathering resistance, good efflorescence protection and low water-uptake, and at the same time an acceptable appearance, needed for coating application on fibre-cement boards.
The single-stage binders in prior-art coatings give access to blocking resistance but are formulated in such a way that significant amounts of solvents or film-forming assistants (coalescence aids) are needed to ensure proper film formation. As the solvents may evaporate into the environment, the low volatile coalescence aids will remain in the coating and may result in reduced mechanical strength. While the two-stage binders of the prior art do provide satisfactory blocking resistance, due to the hard-phase fraction therein, their weathering resistance, however, is often not up to the requirements.